Given this example of a text file containing only some usernames separated by lines:
@Alice @Bob Cindy Leon @Mark How can I add @ to the beginning of every line that doesn't have it already?
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1 Answer
Statements in sed scripts have a [address[,address]][!] action pattern, where address can be a line number or /regexp/ and the address(es) can be reversed with ! (not), so here you'd write:
sed '/^@/! s/^/@/' < file To add a @ to the start of lines that don't already have one.
The address is /^@/ (lines with @ following the start of the line (^)), ! reverses it so that the s/^/@/ action (substitute the start of the line with @) is only run on the lines that don't match.
You can achieve the same effect with:
sed 's/^@\{0,1\}/@/' < file Which replaces 0 or 1 @ at the start with @. Same with EREs:
sed -E 's/^@?/@/' < file answered Nov 18, 2022 at 17:36
Stéphane Chazelas
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- Thank you. Is it a kind of "if" in the first command? Why separated by a space?Sepp A– Sepp A2022-11-18 17:44:34 +00:00Commented Nov 18, 2022 at 17:44
- 1@SeppA see edit.Stéphane Chazelas– Stéphane Chazelas2022-11-18 17:50:32 +00:00Commented Nov 18, 2022 at 17:50
- 1@Quasímodo, yes, though that wouldn't insert a
@if the first bytes of the line could not be decoded into a character in the locale (like on the output ofprintf '\311ric\n'(the iso8859-1 encoding of Éric) if run in a locale that uses UTF-8 (in any case the behaviour of sed is unspecified on non-text input). That would also not add a@at the start of empty line (which may or may not be better).Stéphane Chazelas– Stéphane Chazelas2022-11-19 14:04:06 +00:00Commented Nov 19, 2022 at 14:04